Covert communications in a downlink time division multiple access system

In this paper, we investigate covert communications in a downlink time division multiple access system. While transmitting public messages sequentially to multiple users, the source node concurrently embeds covert messages intended for a covert receiver. Upon reception, the covert receiver cancels the public messages to recover the covert information. We focus on maximizing the achievable sum covert rate by effectively allocating the total transmit power across time slots, subject to a constraint on the detection error probability at the users. The resulting covert rate maximization problem is non-convex with respect to the total transmit power and power allocation variables. To address this, we propose a two-layer optimization framework powered by the gradient projection method. Numerical results validate the effectiveness of the proposed approach and further analyze the impact of key system parameters on the performance.